i. Technical Field
The present invention pertains to improvements in pulp screening apparatus and pertains particularly to an improved efficiency screen basket for use in pressure screens wherein the basket configuration provides a higher level of efficiency than existing designs.
ii. Prior Art
In the papermaking process, screens are used to separate acceptable papermaking fiber from unacceptable constituents of a slurry of pulp fiber, in preparing the pulp slurry to be made into paper. In typical wood pulp screens, a slurry of pulp flows through a perforate, cylindrical screen plate or basket which may be smooth, or which may have a contoured surface facing toward the stock flowing through the screen, to increase the effective screening area. It is known to utilize different hole, slot or hole and slot combinations for optimizing screening performance.
To aid in passing the acceptable pulp through the screen plate, and to reduce plugging, pulsations are generated in the slurry, such as by passing a hydrofoil-shaped member past the screen plate. It is also known to use blunt, turbulence generating members on a screen rotor, to agitate the stock near the screen surface and improve screening efficiency. Such rotors generally operate effectively to screen pulp at one percent to two percent consistency, but do not work effectively at higher consistencies. U.S. Pat. No. 4,855,038 discloses a pulp screen rotor which generates both turbulence and pulse and has been found to work effectively at three percent to four percent consistencies and higher. In many applications, the higher consistencies, referred to as medium consistency, are preferred for process reasons.
To provide sufficient strength to the screen plate, which generally is a basket-shaped member surrounding the rotor, to withstand the pressure experienced in the pressurized screen, and to increase the screening capacity by presenting increased screening area, it has been the accepted practice in all pressure pulp screens to provide a thickly-walled screen plate which is machined to present the desired surface. Such machining is a time-consuming and expensive process. As a result of the manufacturing restrictions in the machining process, which are imposed at least in part by limitations of the machine tools themselves, the total available open accepts flow area in the screen plate has been limited, and the final shape of the screen plate has been a compromise between the limitations of machining and the desired optimum screening shape.
Since the machined screen plates are a single, integral piece, wear or damage in a small area has required replacement of the entire screen plate; including the screening surfaces, mounting surfaces, and support members. When the wear or damage occurs prematurely, the expense of operating such a screen increases significantly.
In co-pending application U.S. Ser. No. 07/206,151, filed June 10, 1988, a modular screen basket is disclosed wherein relatively thin, sheet-like material is formed into the desired shape or contour. Stamping, pressing, and bending techniques are taught to form the screen plate, which does not require machining. The holes or slots are taught to be created during, before, or after formation of the undulations in the sheet-like material, with suggested forming techniques including laser beam cutting and punching. Utilizing a punch die as part of the forming process is suggested as a simplified, cost-effective manner for forming the openings. The modular basket has sufficient strength for pressure screening applications, including medium consistency screening.
In co-pending U.S. application 07/206,151, (U.S. Pat. No. 4,954,249) it is suggested that numerous different undulating configurations, slot and hole combinations and other features can be utilized. The basket forming technique disclosed in U.S. Ser. No. 07/206,151 allows greater control of formation, both of the undulating surface and of the slot or hole openings. It has now been discovered that if various parameters are followed with respect to the physical characteristics of the basket, the basket will operate more efficiency, resulting in optimum screen performance, even at higher consistencies.